<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .flex1{
      display: flex;
    }
    .box{
      flex: 1;
      border: 1px solid #333;
    }
    .box.box-flex2{
      flex: 2;
    }
  </style>
</head>
<body>
  <div class="flex1">
    <div class="box">box1</div>
    <div class="box box-flex2">box2</div>
    <div class="box">box3</div>
    <div class="box">box4</div>
  </div>
</body>
</html>